How to do it...

In order to write variadic function templates, you must perform the following steps:

1. Define an overload with a fixed number of arguments to end compile-time recursion if the semantics of the variadic function template require it (refer to [1] in the following code).
2. Define a template parameter pack to introduce a template parameter that can hold any number of arguments, including zero; these arguments can be either types, non-types, or templates (refer to [2]).
3. Define a function parameter pack to hold any number of function arguments, including zero; the size of the template parameter pack and the corresponding function parameter pack is the same and can be determined with the sizeof... operator (refer to [3]).
4. Expand the parameter pack in order to replace it with the actual arguments being supplied (refer to [4]).

The following example that illustrates all the preceding points, is a variadic function template that adds a variable number of arguments using operator+:

    template <typename T>                 // [1] overload with fixed 
    T add(T value)                        //     number of arguments 
    { 
      return value; 
    } 

    template <typename T, typename... Ts> // [2] typename... Ts 
    T add(T head, Ts... rest)             // [3] Ts... rest 
    { 
      return head + add(rest...);         // [4] rest...  
    }